3.2. Species Composition
A total of 212 phytoplankton taxa were identified during the three cruises in the BS and NYS, belonging to three phyla and 75 genera, and the number of phytoplankton was highest in autumn. Among them, 83 species of 40 genera were identified in spring, 96 species belonging to 43 genera were identified in summer, and 151 species of 62 genera were identified in autumn, all these species belonged to three phyla involving Bacillariophyta, Pyrrhophyta, and Chrysophyta. Bacillariophyta were the most diverse group, and 108 diatom species were identified, Pyrrophyta was the second most diverse group (101 species) and species in Chrysophyta were recorded more sporadically, including three species in one genus. We obtained the top 15 dominant species of the phytoplankton community in each season by calculating the dominance index (
Table 2 and
Figure 3).
In spring, the dominant taxa belonged to diatoms and dinoflagellates in the BS and NYS. Dinoflagellates occurred in low numbers, representing 4.60% of the total cell abundance. Diatoms, chrysophytas, and dinoflagellates were the dominant groups in summer, the most abundant diatom species was
Paralia sulcata. Other species such as
Dictyocha fibula and
Tripos massiliensis f.
armatus, and
Gyrodinium spirale were also relatively abundant. In autumn, no dinoflagellate was among the 15 most abundant species and the diatoms account for 95.85% of total phytoplankton abundance in autumn. The dominant species in the BS and NYS were diatoms, with
Paralia sulcata, Coscinodiscus granii and
Thalassiosira spp. being the common dominant species in the three seasons (
Table 1). In addition,
Dictyocha fibula decreased in autumn compared to summer.
There were 44–56 species among the three seasons in three cruises in all study area. The Jaccard similarity index values ranged between 0.29 and 0.34 (
Table 3), with the highest level detected between spring and summer. This indicates that there were obvious seasonal variations in the phytoplankton community structure. The diversity index and evenness index of phytoplankton community was highest in autumn and lowest in summer (
Table 4).
3.4. Phytoplankton Assemblages Analysis and MDS
The dominance of phytoplankton taxa per season in the BS and NYS is presented in in
Table 2. According to the abundance of the phytoplankton community as biological factors, the phytoplankton community could be classified into 3 ecological provinces based on MDS and cluster analysis (similarity 50%) in each season.
In spring, the first province consisted of 17 stations (
Figure 5) which were mainly located in the BS (
Figure 5). The average value of temperature and salinity were 10.2 ± 1.3 °C and 31.5 ± 1.0, respectively. The total phytoplankton abundance varied between 20 and 4 × 10
3 cells·L
−1, with an average value of 7.03 × 10
2 cells·L
−1. The second province occurred at six stations (
Figure 5). These stations mainly distributed in the coastal areas of NYS (
Figure 5), and the average values of temperature and salinity were 8.9 ± 1.8 °C and 31.8 ± 0.1, respectively. The total phytoplankton abundance ranged from 80–1.81 × 10
3 cells·L
−1, with an average value of 5.63 × 10
2 cells·L
−1. The third province was found at ten offshore stations in NYS (
Figure 5). The mean values of temperature and salinity were 8.1 ± 1.6 °C and 31.8 ± 0.1, respectively. The general phytoplankton abundance varied from 30–3.15 × 10
3 cells·L
−1, and the mean value was 8.47 × 10
2 cells·L
−1. In these three ecological provinces, diatoms dominated throughout the BS and NYS, and accounted for 98.9%, 93.0%, and 92.6% of the total phytoplankton abundance in each assemblage.
In summer, the first province was the same as the first province in spring and consisted by all stations in BS (
Figure 6). The temperature and salinity were recorded with the mean values of 22.7 ± 3.8 °C and 31.3 ± 1.2, respectively. The general phytoplankton abundance was higher than other areas and varied from 1.40 × 10
2 to 7.70 × 10
4 cells·L
−1, with an average value of 5.3 × 10
3 cells·L
−1. Diatoms and chrysophytas accounted for 34.7% and 58.2% of the total phytoplankton abundance, respectively. The dominant species were mainly composed of
P. sulcata,
D. fibula, and
Diploneis bombus. The second province had three stations (N08, N19, N22), as shown in
Figure 6, similar to the second province in spring, and was scattered among the costal of the NYS (
Figure 6). A total of 47 taxa of three phyla, involving 18 taxa of diatoms, 28 taxa of dinoflagellate, one taxon of Chrysophyta were observed. The mean values of temperature and salinity were 17.9 ± 6.1 °C and 31.6 ± 0.5, respectively. The total phytoplankton abundance ranged from 1.60 × 10
2 to 1.15 × 10
4 cells·L
−1 and the mean value was 3.76 × 10
3 cells·L
−1. Diatoms occupied about 73.0% of the total phytoplankton abundance and were dominated by
P. sulcata,
Pseudonitzschia delicatissima,
Meuniera membranacea,
Diploneis bombus, etc. The third province consisted of 12 stations in the NYS as shown in
Figure 6. The temperature showed a drastic difference and varied from 6.9 °C to 27.4 °C. The mean values of salinity (32.0 ± 0.3) were relatively higher than other ecological provinces in summer. The total phytoplankton abundance ranged between 20 and 1.35 × 10
4 cells·L
−1, with an average value of 2.59 × 10
3 cells·L
−1. Diatoms accounted for 80.62% of the total phytoplankton abundance and were dominated by
P. sulcata,
Thalassiosira eccentrica,
Tripos massiliensis f.
armatus.
In autumn, the first province consisted of 12 stations, covering all stations in the BS and one station in the NYS just as shown in
Figure 7. The mean values of temperature and salinity were 18.7 ± 0.6 °C and 31.5 ± 0.5, respectively. The total phytoplankton abundance ranged between 5.71 × 10
3 and 6.40 × 10
3 cells·L
−1, with the mean value of 2.39 × 10
3 cells·L
−1. The second province was composed of six stations and these stations distributed in the coastal area of the NYS. The mean values of temperature and salinity were 18.8 ± 0.4 °C and 31.3 ± 0.6, respectively. The total phytoplankton abundance ranged between 5.71 × 10
3 and 6.40 × 10
3 cells·L
−1, with an average value of 2.39 × 10
3 cells·L
−1. The third province had six stations locating in the NYS, as shown in
Figure 5. The temperature ranged from 10.1 to 20.8 °C, with an average of 16.7 ± 3.6 °C. The average value of salinity was 31.9 ± 0.2. Diatoms were the most dominant phytoplankton group in these three provinces, and accounted for 97.8%, 97.6%, 94.2% of the total phytoplankton abundance, respectively.
P. sulcata and
D. bombus were the dominant species in these three provinces. In addition, B13, N4, and N19 could not be divided into any of the three provinces due to the composition of the top 15 species in these stations having an obvious difference.
The phytoplankton community in the BS and NYS were divided into three ecological provinces in each season (
Figure 5,
Figure 6 and
Figure 7) and the phytoplankton abundance and environmental parameters in the different ecological provinces during the three cruises were shown in
Table 5. In summary, we defined the first provinces in three seasons as a province based on the distribution (in the BS) and defined it as P1. We defined the second province in three seasons as a province based on the distribution (in the coastal waters of the NYS) and called it P2. The third provinces (in the NYS) in three seasons were defined as a single province and we called it P3.
3.5. Vertical Distribution of Phytoplankton Abundance
The vertical distribution of total phytoplankton abundance presented an obvious stratification. According to cluster analysis and MDS, we divided the phytoplankton community of the BS and NYS into P1, P2, and P3, and made corresponding box-shaped diagrams (
Figure 8). For P1, in spring, the phytoplankton abundance was relatively lower in the surface layer, but the abundance in the middle and the bottom layers was the same and higher compared to the surface layer. In summer, the phytoplankton abundance reached its maximum and the phytoplankton community almost distributed in the surface layer. The phytoplankton abundance in the middle layer was almost the same as that in bottom layer. In autumn, phytoplankton abundance did not differ among surface, middle and bottom layers. For P2, in spring, the phytoplankton abundance was lowest in surface layer and highest in the middle layer. The phytoplankton abundance in the bottom layer was lower than in the middle layer but higher than in the surface layer. In summer, the total phytoplankton abundance increased with depth. In autumn, the phytoplankton were mainly distributed in the surface and middle layers, and the phytoplankton abundance in the bottom layer was relatively low. For P3, in spring, the phytoplankton abundance increased gradually from surface layer to bottom layer. In summer, the phytoplankton abundance gradually increased with depth and was highest in the bottom layer. Additionally,
P. sulcata was the dominant specie in the bottom. In autumn, the phytoplankton abundance was same in the surface, middle, and bottom layers.
3.6. Phytoplankton Abundance in Relation to Environment
In order to study the association between environmental parameters and phytoplankton community composition, we did a Canonical Correspondence Analysis (CCA) based on the top 15 dominant species in each season and eight environmental variables (temperature, salinity, depth, nitrate, nitrite, phosphate, ammonia, and silicate). The ordination diagram of CCA exhibits phytoplankton community and environmental variables (arrows) during summer, spring and autumn in the BS and NYS (
Figure 9). Nutrients, depth, temperature, and salinity were the main variables associated with variation in phytoplankton community.
In spring, the abundances of most dominant diatom species were positively correlated with temperature, silicate, nitrate and nitrite concentrations (
Figure 9), while was negatively correlated with depth. However, the abundance of the diatom
Guinardia delicatula was not correlate with any environmental parameters. Dinoflagellates showed positive correlations with temperature and nitrite concentrations. In summer, the abundance of
D. fibula had a strong positive correlation with temperature. Among the 15 dominant species, all diatoms were positively correlated with salinity, depth, and nutrient concentrations and most dinoflagellates correlated positively with the temperature. In autumn, the majority of diatoms showed positive correlations with temperature, depth, and ammonia concentration, whereas a few showed positively correlated with nitrate, nitrite, and phosphate concentrations.